Wireless communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using wireless transmitters and wireless receivers. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal. On the other hand, a receiver is an electronic device which, also usually with the aid of an antenna, receives and processes a wireless electromagnetic signal. In certain instances, a transmitter and receiver may be combined into a single device called a transceiver.
Transmitters in wireless communications devices may often employ mixers to convert a baseband signal at a particular frequency to a radio frequency at another frequency. A frequency mixer is an electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 may be applied to a mixer, and the mixer may produce new signals at the sum f1+f2 and difference f1−f2 of the original frequencies. Mixers are widely used to shift signals from one frequency range to another, a process known as heterodyning, for convenience in transmission or further signal processing. For example, frequency mixers may be used to modulate a carrier frequency in radio transmitters.
The Gilbert cell, such as that depicted in FIG. 4, is often used in frequency mixers. As shown in FIG. 4, a Gilbert cell may include a transconductance stage 2, a mixer stage 4, and an output stage 6. In operation, a differential input baseband signal BBIN applied to transistors 8a and 8b of transconductance stage 2 may generate a differential baseband signal BBOUT at the output of tranconductance stage 2. A differential mixer frequency signal VLO applied to transistors 10a, 10b, 12a, and 12b of mixer stage 4 as depicted in FIG. 4 may modulate the baseband signal BBOUT, thus generating a differential signal VRF. The differential signal VRF may be further conditioned by a cascoded common-gate current buffer formed by transistors 14a and 14b of output stage 6, and balun 16 with tuning capacitances 18 and 20 of output stage 6, with the Gilbert cell producing an output signal Vout.
A direct current (DC) bias present on the output of transconductance stage 2 can be optimized to provide for signal headroom for both transconductance stage 2 and mixer stage 4, with the respective headrooms of transconductance stage 2 and mixer stage 4 trading off. However, the DC bias may vary due to variations in process, voltage, and/or temperature, leading to non-linearity of the Gilbert cell. Also, even-order harmonics generated by transconductance stage 2 may further degrade linearity of the Gilbert cell via direct up-conversion by mixer stage 4 and/or intermodulation.